The most basic services provided by a digital trunking communication system comprise group call service and broadcasting service. The group call service allows a trunking user terminal establishing a group call to a certain group of trunking user terminals belonging to a given area, and with the half-duplex mode, all the trunking user terminals within the group in a same cell share one downlink and pre-empt the uplink; only the calling party in the broadcasting call service can speak, the others are listeners, and if the calling party exits the call, the broadcasting call terminates. The common feature of these two services is: users having no speaking right in the group call have no Radio Resource Control (RRC) connections, no dedicated channels, and jointly monitor the group downlink broadcast channel; a user having/not having the speaking right in a single call and the user having the speaking right in the group call are based on the RRC connection, have dedicated channels, and are similar to general Long Term Evolution (LTE) users.
Because of the time-varying performance of the wireless channel, the traditional LTE technology uses the Adaptive Modulation Coding (AMC) mode to make the system adaptively tracking the link change by dynamically changing the coding scheme and modulation order: when the channel condition is relatively good, a relatively high order MCS will be used for data transmission to improve the system throughput; when the channel condition is relatively poor, a relatively low order modulation and coding scheme will be used for data transmission to ensure transmission reliability. The method of a traditional LTE implementing adaptive modulation and coding is: the terminal measuring the downlink channel quality to obtain a downlink signal to interference plus noise ratio (SINR) value, the terminal queries the SINR and CQI mapping table to obtain a downlink channel quality indicator (CQI), and uses the uplink PUCCH (Physical Uplink Control Channel) or PUSCH (Physical Uplink Shared Channel) to report the obtained CQI to the base station, the base station queries the CQI and MCS mapping table to obtain an MCS inner-ring value according to the reported CQI; the base station calculates a Block Error Ratio (BLER) in accordance with a downlink hybrid automatic repeat request (HARQ) result reported by the terminal, and then calculates the MCS outer-ring value through the sampling value of the BLER.
A digital trunking communication system based on conventional LTE technology achieves the traditional trunking service. Since the conventional LTE technology is designed for point-to-point communication services, while the trunking service is a point-to-multipoint service, the trunking service characteristics determine that the MCS used in the trunking data transmission is also different from that used in the general LTE technology, and the performance is as follows:
the downlink data transmission of a trunking user does not have HARQ feedback, so that the base station cannot be informed the packet error rate of the terminal and cannot obtain the MCS outer-ring value; users having no speaking right in the trunking do not have uplink channels, thus the users cannot use the process of a general LTE user reporting the CQI; the group users are separated, the distribution is relatively random and must ensure that each user in the group can reliably demodulate the encoded information; each end-user in the group will report the CQI, and the base station must reasonably process the CQI information reported by a plurality of terminals in the group.
These characteristics determine that the modulation and coding method used in the current LTE is not applicable to a digital trunking communication system based on the LTE technology.